The present invention relates to a thermal photodetector such as a highly sensitive infrared detector or the like to be used in a Fourier transform infrared spectroscopy (FT-IR) or the like.
As the highly sensitive infrared detector of the room-temperature operating type for FT-IR, a pyroelectric infrared detector is used. Particularly, there is widely used a pyroelectric infrared detector using a pyroelectric crystal of the TGS (triglycine-sulphate) system such as D-TGS, LA-TGS, DLA-TGS or the like which presents a great pyroelectric coefficient.
A pyroelectric element is used in a highly sensitive infrared detector because of the following reasons. In a quantum-type infrared detector element using a semiconductor or the like, the band gap is small so that the influence of a dark current is great. Accordingly, no good sensitivity can be obtained unless such an element is cooled by liquid nitrogen. Further, the wavelength range which can be detected by such a highly sensitive element is limited to a wavelength range shorter than 14 .mu.m.
In TGS-system crystals, the Curie temperature Tc is about 60.degree. C. even for DLA-TGS of which Curie temperature is the highest in the TGS-system crystals. In such a crystal, the temperature range in which the sensitivity is stabilized is as narrow as 24.degree. to 36.degree. C. It is therefore inevitably required to control the temperature of a pyroelectric crystal. A pyroelectric infrared detector is included in a thermal photodetector adapted to detect fine temperature change due to the incidence of infrared radiation (heat wave) upon the pyroelectric crystal. Accordingly, if the pyroelectric crystal is cooled in the same manner as done for a quantum detector element, this produces an adverse effect such as decrease in sensitivity, mixing of noise or the like.
More specifically, in a quantum-type detector element, there is adopted a cooling method for only the purpose of efficiently liberating the heat quantity of the element in order to maintain the element temperature to a predetermined temperature or less. However, when such a cooling method is merely applied to a pyroelectric detector element adapted to detect a heat energy itself, the detector element is lowered in responsivity. In the worst case, there are instances where the detector element cannot detect a heat energy any more due to noise caused by temperature control circuit.
In this connection, provision is conventionally made as set forth below. Although the performance of FT-IR apparatus is lowered in its entirety, the optical system of the FT-IR is stopped down so that the crystal temperature does not exceed 36.degree. C. even at the time when the amount of light incident upon a pyroelectric detector is maximized. Alternatively, the pyroelectric detector is indirectly adjusted in temperature from a slightly remote place.
FIG. 5 (a) to (b) show a conventional example of such a temperature adjusting structure. In this structure, a temperature adjusting medium 52 is sticked to the periphery of a lateral wall of an enclosing body 511 which encloses a pyroelectric detector 51, and a Peltier element 53 serving as temperature adjusting means is disposed at the other end of the adjusting medium 52. Accordingly, the temperature of the pyroelectric crystal of the pyroelectric detector 51 is indirectly adjusted.
However, highly precise and stable temperature control cannot be always assured by the arrangement shown in FIG. 5 (a) to (b) for the following reasons. That is, a plurality of detector assemblies each having the temperature adjusting structure shown in FIG. 5 (a) to (b), differ from one another in the thermal conductivity and thermal time constant between the detectors and the detector enclosing bodies 511. Further, the thermal contact areas between the pyroelectric detector 51 and the medium 52 and between the Peltier element 53 and the medium 52 are great. Accordingly, a plurality of detector assemblies differ from one another in thermal contacts (thermal resistances) between the pyroelectric detectors 51 and the media 52 and between the Peltier elements 53 and the media 52. Thus, the respective detector assemblies are poor in reproducibility.
On the other hand, the following examples are conventionally proposed of a structure arranged with the improvement in sensitivity of a pyroelectric detector taken into consideration (a structure in which heat escape from a pyroelectric element is minimized). As shown in FIG. 6 (a), a support base plate 62 of a pyroelectric element 610 has an opening 621 formed by etching, so that the underside of a light receiving portion (where electrodes 611 are formed) is hollow to prevent thermal diffusion. As shown in FIG. 6 (b), a pyroelectric element 710 is placed on a frame 72 having four legs to prevent thermal diffusion.
When such a structure is applied to a pyroelectric detector element using a TGS-system crystal, the element temperature cannot be controlled in a predetermined range so that the element cannot be operated in a normal manner. To operate a pyroelectric detector element using a TGS-system crystal with good sensitivity, it is required to thermally connect the element to the temperature adjusting medium in a suitable amount. In this connection, such a thermal connecting medium should have a highly precise structure with good reproducibility.
According to the structure shown in FIG. 6 (a), the base plate 62 is made of MgO or the like and the opening 621 is formed by etching the base plate 62 with phosphorus. However, the opening thus formed by etching is poor in dimensional precision.
To make the structure shown in FIG. 6 (b), there is required a step of placing a minute pyroelectric element (having sizes of about 3 mm.sup.2 and a thickness of about 10 .mu.m) 710 on the frame 72 having four legs. It is not easy to automatically carry out such a step without the element damaged. Thus, the structure shown in FIG. 6 (b) is not fit for mass-production.